In a cardiac pacemaker, also called a cardiac pulse generator, it is usual to define a base frequency (or basic rate) at which stimulation pulses are delivered in the absence of a spontaneous cardiac rhythm. The base frequency value is typically programmed. It is understood that a base frequency in hertz or beats per minute has a corresponding time interval in seconds, usually expressed in ms. The base interval also is referred to as a basic escape interval ("EI"), which interval starts at a first cardiac event, and results in a stimulation pulse being delivered at the end of the interval unless a spontaneous cardiac event occurs before the end of the interval.
As appreciated by the inventors, during periods when the patient is at rest, a spontaneous cardiac rhythm can exist which is below the base frequency, and which is physiologically compatible with the state of the patient. A problem which exists with conventional pacemakers, however, is that the regulation of the device at the programmed base frequency will not be able to accommodate such a spontaneous rhythm below the base frequency.
Several attempts have been proposed to vary the base frequency function during periods of rest and activity of the patient.
U.S. Pat. No. 5,143,065 describes a predictable model, in which the base frequency is adjusted as a function of the time of day and cyclic physiological needs of the patient.
The document WO 86/07270 describes a function simulating the day and night variations (circadian rhythm) of the natural cardiac rhythm.
None of these documents, however, allows the adaptation of the base frequency to periods of real activity or effective rest of the patient.